On September 16, 1862 Maj. Gen. George B. McClellan and his Union Army of the Potomac confronted Robert E. Lee’s Army of Northern Virginia at Sharpsburg, Maryland.

At dawn on September 17, Maj. General Joseph Hooker’s Union corps mounted a powerful assault on Lee’s left flank that began the single bloodiest day in American military history.

Repeated Union attacks, and equally vicious Confederate counterattacks, swept back and forth across Miller’s cornfield and the West Woods. Despite the great Union numerical advantage, Stonewall Jackson’s forces near the Dunker Church would hold their ground this bloody morning.

Meanwhile, towards the center of the battlefield, Union assaults against the Sunken Road would pierce the Confederate center after a terrible struggle for this key defensive position. Unfortunately for the Union army this temporal advantage in the center was not followed up with further advances.

Late in the day, Maj. General Ambrose Burnside’s corps pushed across a bullet-strewn stone bridge over Antietam Creek and with some difficulty managed to imperil the Confederate right. At a crucial moment, A.P. Hill’s division arrived from Harpers Ferry, and counterattacked, driving back Burnside and saving the day for the Army of Northern Virginia.

Despite being outnumbered two-to-one, Lee committed his entire force at the Battle of Antietam, while McClellan sent in less than three-quarters of his Federal force. McClellan’s piecemeal approach to the battle failed to fully leverage his superior numbers and allowed Lee to shift forces from threat to threat.

During the night, both armies tended to their wounded and consolidated their lines. In spite of crippling casualties, Lee continued to skirmish with McClellan on the 18th, while removing his wounded south of the Potomac. McClellan, much to the chagrin of Abraham Lincoln, did not vigorously pursue the wounded Confederate army.

While Antietam is considered a draw from a military point of view, Abraham Lincoln and the Union claimed victory. This hard-fought battle, which drove Lee’s forces from Maryland, would give Lincoln the “victory” that he needed before delivering the Emancipation Proclamation - a document that would forever change the geopolitical course of the American Civil War.